Conventionally, there has been a user/machine interface including a panel that has a surface and can sustain flexural waves, a touch-sensitive input device that has the surface associated, and a unit that includes a force transducer to give force feedback to the input device. The force is given in a format of pulses on the panel, the pulse is given in a format of a modulation signal that presents a sense of button-clicking at the fingertip of a user, and the modulation signal has a base carrier frequency in the range of 150 to 750 Hz and having the duration of at least 10 ms (see, for example, Patent Document 1).